Predicting U.S. Supreme Court decisions by listening to oral arguments is notoriously hazardous. But perhaps the most telling sign in today’s historic arguments in the cases of Jose Padilla and Yaser Hamdi, two U.S. citizens whom the White House has labeled “enemy combatants,” came unexpectedly when even Justice Antonin Scalia, the court’s most forceful conservative, seemed to challenge the Bush administration’s claim of seemingly unlimited powers during wartime.

“Where does he get that power?” Scalia asked Deputy Solicitor General Paul Clement to explain at one point.

The power of a president to wage war, Scalia said, is like “the George Washington-like” power to command armies in battle. “It doesn’t mean he has the power to do whatever it takes…”

Scalia’s question was just one of the many sharp questions justices asked about the broad claims that the administration has asserted in the so-called “enemy combatant” cases. These cases may pose the most significant civil liberties issues to come before the high court in decades, raising fundamental issues about executive powers and citizens’ rights in the new age of the war on terror.

Hamdi is a Louisiana-born U.S .citizen who was picked up by the pro-American Northern Alliance while allegedly fighting for the Taliban and handed over to U.S. forces in Afghanistan in the fall of 2001. Padilla is a Brooklyn-born former street gang member who was arrested at Chicago’s O’Hare Airport in May, 2002. U.S. officials claimed they had secret intelligence that he was plotting to assemble a radiological “dirty bomb” – and Clement today compared him to a “latter day, citizen version of Mohammed Atta,” the lead 9/11 hijacker. In both cases, the Bush administration has asserted the rights to hold both men–without even granting them the right to consult lawyers–so long as “the war” lasts.

But how long is that? the justices wanted to know.

The question was premature, at least with respect to Hamdi, Clement said, given there were still 10,000 troops in Afghanistan more than two-and-a-half years after Congress first authorized Bush to respond to September 11–a clear sign that the conflict was continuing.

But Justice Sandra Day O’Connor asked “if the war-like situation could last 25 years?”

“With respect to Al Qaeda, the end of the war is difficult to say,” responded Clement.

“Let’s say it’s the 100 years war?” retorted Justice Stephen Breyer. And if it is “the 100 years war,” Breyer added a moment later, does that mean that “there is no opportunity for the courts to say” at any point that the violations of Hamdi and Padilla’s civil liberties can never be challenged?

But the justices also indicated at times they were troubled about where to draw the line. When Padilla’s lawyer, Jennifer Martinez, argued that he should either be charged with a crime or let go, O’Connor worried about the implications.

“What do we do?” she asked. “Just let loose a ticking time bomb?”

Many of the questions seemed to revolve around exactly what legal basis the administration was making for its extraordinary claims of power. In both cases, the administration has pointed to a Congressional resolution passed one week after September 11 authorizing the president to use “all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed or aided the terrorist attacks.” That means, Clement argued today, that if those “persons” happen to include U.S. citizens, then “all necessary and appropriate force” would include locking them up for as long as military needs dictate he do so–at least until they can be interrogated so the government can glean whatever “intelligence” they may posses to ward off future attacks.

But Martinez, Padilla’s lawyer, challenged the idea that Congress ever contemplated, in the days after September 11, the idea of picking up U.S. citizens off the streets and detaining them without trial. “There is no indication that Congress thought they were authorizing indefinite detentions, she said. Only a few months later, while considering the Patriot Act in the fall of 2001, Congress had extensive debates over how long to authorize the detention of non-U.S. citizens without trial. Surely, she contended, Congress could not have envisioned granting fewer rights to U.S. citizens.

The government “cannot take citizens off the streets and lock them up forever without a trial,” Martinez concluded. “The government asks in this case for basically limitless power. Never before in history has this court granted a President a blank check to do whatever he wants to American citizens.”

From the outset, some administration lawyers were worried that the White House’s hardline stand in the Hamdi and Padilla cases was too uncompromising–and was setting Bush up for a politically embarrassing election year defeat by the Supreme Court. At the very least, these lawyers argued, the White House should allow both men to have a chance to meet with their lawyers. In recent months, the administration relented and allowed closely monitored, videotaped visits with their lawyers–but as a matter of “discretion” rather than an acknowledgement of their legal rights.

But that small concession seemed unlikely today to make much of a difference. The justices could, as they sometimes do, carve out a nuanced and split verdict: The Court could reject a U.S. Court of Appeals opinion in the Padilla case that completely dismissed the president’s claims that he ever has the right to detain U.S. citizens absent specific authority by Congress–but still try to impose some limits on the powers the president is asserting. It could also rule that the specific circumstances in Hamdi’s case–he was allegedly picked up on the battlefield carrying an AK-47 rifle–make a strong circumstantial case that he deserves the “enemy combatant” label.

But, if the questions were any guide, the one scenario that seems least likely is that the Supreme Court will grant what the president seems to be asking for: a blank check.